theorem
  for f1 being Function of X1,D1 for f2 being Function of X2,D2 holds [:
  f1,f2:] = <:f1*pr1(X1,X2),f2*pr2(X1,X2):>
proof
  let f1 be Function of X1,D1;
  let f2 be Function of X2,D2;
  dom f1 = X1 & dom f2 = X2 by FUNCT_2:def 1;
  hence thesis by Th66;
end;
